1. Field of the Invention (Technical Field)
The present invention relates generally to aerospace. More specifically, the present invention relates to a diaphragm valve designed for thin wall tank locations. The valve controls flow of cryogen through the tank wall. One application is cryogenic propellant transfer in reusable launch vehicles.
2. Background Art
Liquids are transferred in many industries. For example, cryogenic propellants are used in aerospace. The space launch industry offers a unique set of cryogenic liquid transfer challenges and requires innovative valve hardware techniques to accomplish the safe and cost effective transfer of liquids. The valve should have reduced mass, bulk, volume, maintenance and cost.
Originally, super cooled liquids used in expendable launch vehicles (ELVs) took days to transfer from storage to the launch vehicles prior to launch from the pad. The process starts with a precooling of the receiving tank, and proceeds through difficult methods of determining the completion and methods to compensate for the boil off of the cryogenic liquid. The entire tank is filled and drained on the launch pad sometimes several times.
Liquids may also be transferred once in orbit. The traditional integrated cryogenic liquid propellant loading process in the microgravity of orbit between two large tanks is complicated by traditional valves, cryogenic tank pre-cool down requirements, and the lack of industry changing valve innovation.
The lunar transportation is expected to start from low earth orbit with the tankage being filled using expendable tanks, transported to low earth orbit in various tanks sizes to reusable space transportation vehicles in low earth orbit.
The transportation of cargo to space is expensive. Part of the problem is the high cost of the individual operations required for the transport of cargo to orbit. A new valve could reduce cost in many ways through innovation, such as using a different sealing method, using fewer valve wear surfaces, and installing the valve in the tank wall.
Since the beginning of space travel, thin wall propellant tanks have presented a problem for engineers incorporating interfaces to fill, vent, and drain the contents. The weight of propellant tanks can be optimized if the loads on it are distributed over a large region. Interface points for mounting the tank mass to a vehicle tend to distribute the loads fairly well. Interfaces for fluid piping connections tend to be point load locations on these tanks and must withstand substantial loads due to cryo shrinkage and ground systems. The moving mass of liquid has a momentum related to it and the control of the flow also results in additional loads on the tank and traditional style valves. Over the past several years, investigations into low cost cryogenic tankage for space applications have identified the need for cryogenic valves with near zero leak-rate and the capability to be packaged into the wall of propellant tanks. These valves could reduce the point loads on the tanks by becoming flatter and distributing their loads over a broader area of the tank wall. Moreover, dynamic loads caused by mounting valves on pipes or ducts that extend from the tank for the sole purpose of mounting a valve could be eliminated.
This liquid transfer control problem has been partly addressed by various minor changes in valves for centuries and patch valve improvements to the thirty-year-old facilities used to launch early expendable launch vehicles (ELV's) through the Space Shuttle. These incremental improvements are helpful, but not enough for the reusable vehicles of the future to bring down the cost of operations. Future generations of launch vehicles called the reusable launch vehicles (RLVs) are fully reusable and need robotic transfers of cryogenic liquid tanks with thin wall valves.
Cryogenic systems have been used in aerospace applications to provide propellant required to launch mass into earth orbit. Now these systems must operate without human onsite assistance beyond earth orbit to the lunar surface and beyond. Cryogenic systems have also been used advantageously in various scientific and technical applications including gas chromatography, superconductivity, magnetic resonance, and medicine. For example, in medical applications, cryogenic systems have been used for ultra rapid cooling of biological samples. In the field of gas chromatography, cryogenic focusing systems have been used to thermally trap analytes before injecting the analytes onto a column. A cryogen (such as liquid oxygen and liquid hydrogen provide maximum energy per weight unit) maintained at a low temperature is introduced for a controlled period of time to cool a cryogenic trap.
In the past, motor actuated valves (e.g., U.S. Pat. No. 3,898,863), float type valves (e.g., U.S. Pat. Nos. 4,873,832, 4,592,205 and 4,607,489), and solenoid valves (e.g., U.S. Pat. No. 4,348,873) have been used in cryogenic applications. In aerospace applications the absence of valve stems, maintenance, robotic operation and the elimination of piping are important.
Certain types of diaphragm valves also have been used for cryogenic liquids and other liquids (e.g., U.S. Pat. Nos. 2,638,127 and 4,086,784). However, these have been used as pressure modulated valves, where the valve is opened on the pressure of the cryogen.
Generally, solenoid valves are rated at approximately one million cycles and are subject to failure when operated substantially beyond one million cycles. Failures primarily are related to the effects of low temperature (i.e., −196 degrees C. for liquid nitrogen). Solenoid valves are also subject to leakage, slow response, lack of reliability at low temperatures and/or low flow rates, and wear of contacting surfaces. Solenoid valves tend to stick when operating at low temperatures. Solenoid valves also have not been capable of opening and closing at a sufficiently high frequency which is necessary to achieve a high degree of control of liquid nitrogen flow.
For solenoid valves in aerospace applications beyond earth's gravity, a number of otherwise suitable materials become brittle at low temperature. Materials must perform satisfactorily over the complete range from cryogenic to room temperature in a variety of gravity situations. Stresses due to thermal contraction, and radial, axial and circumferential temperature gradients caused by non-uniform cooling rates are frequently encountered in the cool-down of cryogenic transfer lines and valves associated therewith. For example, seals at the center of the plunger in the valve harden at low temperature, resulting in leakage and/or blow-by across the solenoid valve.
To improve the reliability and effectiveness of solenoid valves in aerospace, attempts have been made to reduce critical tolerances, use more advanced materials, and use redundant parts for sealing, such as multiple O-rings. However, such designs are prohibitively expensive for many applications.
In the past, solenoid valves capable of handling high pressures (i.e., above approximately 100 psi) have been limited to low flow rates. Aerospace needs relatively low pressures compared to other industrial applications and higher flow rates. Typically, a solenoid operated cryogenic valve operates against a pressure/area differential, and the electromagnet is therefore a limiting factor on pressure. Moreover, solenoid valves used in cryogenic applications have slow response times due at least in part to increased electric current requirements at low temperatures.
Examples of cryogenic liquid transfer from other industries include the optical coating industry and methane tank service industry. U.S. Pat. No. 3,938,347 to Riedel, et al., assigned to Optical Coating Lab, entitled “Level Control Apparatus and Method for Cryogenic Liquids” discloses cryogenic tanks, with level sensing means in the form of thermocouples and resistors inside the tank.
Cryogenic liquid filling and ullage in the methane industry is typically accomplished by placing instrumentation in the tank. For example, U.S. Pat. No. 4,334,410 to Drumare, entitled “Tank Designed to Contain a Liquefied Gas” discloses a system that uses methane tanks, which work at minus 160 degrees C. and typically get 98% full or 2% ullage. In filling methane tanks the heat responding device and the temperature responding element are placed inside the tank and are a source of extra expense. Each element of complexity is a source of failure.
Typically, the ground operations with cryogenic valves include the handling, monitoring, control and effective use of all the liquids required for the transportation cycle to orbit. The total cost of the transportation cycle is in part the result of the ground operations. The cost-effective transfer of vehicle propellants at the ground facility could be enhanced by an improved diaphragm valve, and the efficient use of the reusable vehicle and its propellants.
New valve technology is therefore needed to overcome the problems described above.